What is soil compaction?
Soil compaction is a physical degradation of the soil caused predominantly by agronomic management. In arable land, both topsoil and subsoil compaction is possible.
- Compaction results in an increase in bulk density of soil resulting from an applied force.
- Surface compaction caused by livestock is generally shallower than 150 millimeters and is commonly rectified during sowing.
- Subsurface compaction is generated through the application of load by vehicular traffic. The depth can vary but has been measured greater than 500 millimetres below the surface.
- Soil compaction differs from hard layers formed through natural processes, such as chemical cementation, transient bonding and dispersion.
Soil compaction occurs when soil particles are packed closer together. Once compacted, soil becomes stronger, denser and less porous, and holds less water and air, slowing root growth and inhibiting both water and nutrient uptake. Although some soils are naturally compact, soil compaction is generally due to repeated pressure from agricultural machinery and stock ‘squashing’ the soil profile when wet.
Overview of soil compaction in Western Australia
Effective management of soil compaction, especially subsoil compaction, is important for improved water and nutrient use efficiency. From Soil Quality: 6 Soil Compaction (Parker et al. 2021). Video talent: Wayne Parker, DPIRD; video production: Lomax Media.
Causes of soil compaction
Soil compaction occurs when the applied forces of compression, shearing and smearing cause the rearrangement of soil particles, resulting in a reduction in total pore space, increased bulk density and soil strength. With an increase in soil bulk density, the same mass of soil fills a smaller soil volume. Changes in bulk density do not always signify compaction; bulk density increases naturally down the soil profile, with lower organic matter, less root penetration and less pore space compared to surface layers.
For plants to grow in a paddock, germinating plants must be able to drive their shoots and roots through the soil. As soil strength increases, the ability of the plant to extend or grow roots is restricted. Soil strength is influenced by the natural characteristics of the soil and agricultural management practices.
Heavy machinery
In coarse textured sandy soil, the main cause of subsoil compaction in agricultural areas is trafficking of farm machinery. Trafficking causes soil packing under wheels, with the resulting hard layer often referred to as a ‘traffic pan’. Subsurface soil compaction is common in uniform, coarse textured sandy soil with soil texture ranging from a loamy sand (5–10% clay) to a sandy loam (< 20% clay). In finer textured clay soils, subsoil compaction may also be associated with tillage practices undertaken whilst soil is wet and the formation of a ‘plough pan’. In this case, tilling soil repeatedly at a constant depth creates a dense layer below the cultivation depth where tillage implements have smeared soil particles. This ‘plough pan’ effect can also be observed under deep strategic tillage operations in coarser soil types, such as ploughing or spading.
Traffic by farm machinery and repeated cultivation by tillage implements force soil particles and aggregates closer together by the downward pressure from farm machinery (compression) as well as the spinning or slipping of wheels and the action of tillage implements causing sideways force and realignment of the soil particles (shearing, smearing). As a result of these forces, the same mass of compacted soil occupies less volume (increased bulk density) with fewer and smaller pore spaces (reduced porosity) compared to uncompacted soil.
Livestock
As both machinery and livestock are supported by the soil, their ‘footprints’ alter soil structure. As with machinery, the relative impact of livestock differs with weight, movement, speed, size and time spent in contact with the soil – as well as grazing pressure. In the same way the passage of a machine negatively impacts soil strength and bulk density, soil porosity and water infiltration – grazing livestock alter the soil surface and can cause a level of near-surface compaction, restricted to a maximum of 150 millimetres depth. This livestock-induced compaction occurs predominantly around traffic areas, such as paddock trails, gateways, camps and water points. The highest levels of compaction associated with livestock occur when:
- soil is wet
- ground cover is poor
- soil is poorly structured.
Research has determined that soil strengths in the surface levels of a grazed soil are rarely above levels which will restrict root growth (2–3 MPa). This near surface, relatively low level compaction is often alleviated during sowing with the tillage point providing sufficient fracturing of the hard layer – effectively removing any impediment to root growth.
Livestock and machinery trafficking both act to decrease soil porosity in the surface which can lead to increased waterlogging of prone soils during wet periods. Much of this is induced during periods of summer grazing as livestock powder the surface and finer soil particles clog soil pores, decreasing macro-porosity and movement of water resulting in less aerated soil. Soil surface crusting can result, with sealed pores decreasing the infiltration, or entry of water into the soil profile. As water is unable to freely enter the soil, it will pool and/or flow via the path of least resistance leading to increased water run-off, erosion and degradation of soil quality. Less percolation of water into soil also means less soil moisture available to support crop and pasture production, and less groundwater recharge.
Chemical soil properties
In addition to compaction, hard layers can also form throughout the soil profile as a result of natural soil packing and chemical cementation processes (see soil stability). Chemical cementation and transient bonding reduce pore space within the soil matrix, without mechanical compaction of the soil particles. These layers can be hard and restrictive when dry but may vary in hardness on wetting. If the cementation process continues long enough, hard layers may transform into permanent duricrusts.
For effective management, it is necessary to distinguish between mechanical compaction and other processes. Hard soil layers resulting from natural soil processes can often be fully or partially alleviated with deep tillage to a depth greater than the hard pan. Soil naturally prone to the development of hard layers may benefit from the use of soil amendments such as gypsum, with organic matter needed to stabilise changes in soil porosity. However, in situations where subsoil sodicity is high, deep tillage may, in fact, make the situation worse and should be avoided.
Soil dispersion in sodic soil can induce a hard layer where clay particles disperse into water on wetting, causing the collapse of soil aggregate structures and ‘slumping’ of the soil. Dispersion can be exacerbated by tillage, which contributes to the destabilisation and breakdown of stable soil aggregates.
These hard layers can be present in both surface and subsurface soil and differ from compaction as they are chemically, not physically, induced. However, there are many examples of interactions between these mechanisms in broadacre cropping.
Hard layers in soil: compaction or chemical cementation
Processes forming hard layers include compaction, where soil pore space sediments is decreased by the weight of traffic and pressure of overlying layers, and cementation, which results from sediments being 'glued' together by new minerals deposited by water and hardening. From Soil Quality: 6 Soil Compaction (Parker et al. 2021). Video talent: Matthias Leopold, University of Western Australia; video production: Lomax Media.
Factors influencing soil compaction
Soil bares the weight of multiple passes of large machinery that are required to sow, manage and harvest a crop each season. Soil components and the load it bares influence the severity of compaction induced throughout the profile.
- Moist soil increases severity.
- Uniform distribution of soil particle sizes increases susceptibility.
- Good, stable structure reduces susceptibility.
- Clay mineralogy may help with self-repair.
- High levels of organic matter (>3% organic carbon) can minimise risk and severity.
- Any vehicular traffic across a soil surface will induce some level of compaction. The severity and depth of compaction depends on the total load, its distribution, frequency of traffic, speed of operation and the area of contact with the soil.
These factors are described further below.
Most soil is susceptible to compaction, but aspects of overall soil quality and moisture status can contribute to some being relatively more susceptible. The two most important factors for susceptibility to compaction are soil moisture and particle size distribution.
The risk of compaction is greatest in fine-textured and low organic matter soils, some of which are naturally compacted. For example, clay soils are more naturally compact than sandy soils, and often become less stable when wet. Shrinkage cracks in dry soils can indicate a more ‘resilient’ soil, able to naturally disrupt compacted layers by alternately swelling (when wet) and shrinking (when dry). In some cases, cracking clays can also demonstrate a dense structure, so that “self-mulching” types of these soils are considered desirable. Sands are readily compacted by traffic.
Compaction is more problematic in coarse textured sandy soil compared to soil with higher loam or clay content. Clay-dominant soil restricted by a compaction layer has higher capacity to store plant-available water content in the top 300 millimetres when compared to a sandy soil with a compaction layer at the same depth. In sand, plant roots need to explore deeper in the soil to extract the same volume of water, so are likely to gain a proportionally larger plant growth response post amelioration from accessing deeper water (presuming no other constraints are present).
Moisture content
Dry soils compact less than wet ones, and clay-rich soil is more likely to be damaged through compaction and/or remoulding when the soil is wet compared to sandy soils.
Soil moisture content at the time of wheel load compression influences the level of compaction. This interaction is compounded by soil texture, with increasing fineness of soil increasing susceptibility to compaction. However, each texture has its own optimum water content where the risk of compaction is highest. Sandy textured soils are most susceptible to compaction at 12–15% water content, which is close to their field capacity (the amount of water a soil can hold against gravity). A loamy soil would need more water to become susceptible to compaction. As moisture content increases, the water acts as a lubricant for soil particles to arrange themselves under pressure closely in an ‘optimal’ dense distribution, increasing the severity of compaction.
As a rough guide if you can form a 3 mm diameter rod by squeezing soil in your hand (field moisture content), then the soil is too wet for cultivation.
Particle size distribution
Particle size distribution of a soil influences how the particles interact under applied pressures, making the soil more or less susceptible to compaction.
Loamy sand, clayey sand and sandy loam soil have high susceptibility to compaction due to a more even distribution of sand, silt and clay – allowing particles to pack more closely.
Sandy clay loams, clay loams and clays have low susceptibility to subsurface compaction.
While sandy soil has a relatively uniform particle size, there may be resistance to root growth even without an obvious compaction layer due to the interlocking nature of angular soil particles. However, the older the sand particles the more rounded they are and, thus, more likely to reach optimum compaction.
Soil with a high (> 50%) gravel (> 2 mm) content better withstand heavy machinery, and while localised areas of the soil matrix between gravel stones can still compact, the gravel matrix often provides pathways through which roots can continue to grow.
Structure and structural stability
Good and stable soil structure minimises the risk of soil compaction. The integrated nature of organic and mineral particles, as well as bacteria and fungi, work to hold the solid particles longer and maintain soil structure under pressure.
Clay mineralogy
Shrink-swell properties of some clay minerals such as smectite, illite or vermiculite may cause damage to crops due to ripping apart fine roots. However multiple wettting-drying cycles may self-repair compaction that was caused under moist soil regimes.
Organic matter content
Soil organic matter plays a central role in many physical properties of the soil including aggregate formation, water holding capacity and structural stability. Soil high in organic matter demonstrates increased resilience, making it better able to recover from stresses, including compaction, compared to soil low in organic matter content.
In the absence of multiple traffic passes, good soil structure and higher levels of organic matter (> 3% soil organic carbon) can minimise the risk and degree of compaction. In the south-western agricultural region of Western Australia, organic matter is usually highest in the topsoil and often declines rapidly with depth. With 80% of agricultural soil in the south-western agricultural region having soil organic carbon levels between 0.5 and 2% in the surface (0–100 mm), and with the exception of permanent pasture systems, it is likely that organic matter has little influence on the formation of subsurface compaction.
The efficacy of deep ripping has, in some instances, been prolonged where organic matter has been added into subsoil, with soil structure maintained for longer, prior to natural or machinery-induced re-compaction. The ‘living’ organic fraction (soil fungi, bacteria and plant roots) also help stabilise soil aggregates.
Any vehicular traffic across a soil surface will induce some level of compaction. Soil carries the weight of the vehicle at the points of contact. The contact pressure exerted by the tyre or track on the soil surface is a function of both the vehicle weight and area of the tyre in contact with the soil surface.
Compaction severity is related to the contact pressure of tyres with the ground, whilst extent relates to the area of soil affected by traffic. Aim to reduce the trafficked area.
The severity and depth of compaction depends on the total load, its distribution, frequency of traffic, speed of operation and the area of contact with the soil. During machinery tracking, the weight of the vehicle is transferred down through the soil. Greatest stress occurs at the point of contact between tyre and soil and diminishes down the profile. This increases soil strength throughout the layer that incurs these stresses and is seen as a bulge in soil strength readings. The depth of the peak in the bulge is dependent on vehicle weight.
Total load is the primary determinant of soil compaction – a heavier load results in more severe compaction to a greater depth. Current tractor weights of 25 to 30 tonne, equate to an axle load of up to 15 tonne per axle, or 7.5 tonne per tyre, before the tractor is work-loaded. On sandy soil, these high axle loads often result in a measure of soil strength in excess of 2.5 megapascal (MPa) at depths of up to 550 millimetres deep – reflecting a higher bulk density in compacted soil. Compaction layers reaching these soil strengths have been determined to slow, or even stop root growth, preventing access to deeper moisture and nutrients.
Depending on soil conditions and load, a single traffic pass may increase soil bulk density by 75%, with an increasing number of passes continuing to increase bulk density further. On uniform, course textured sandplain soil, there is a relatively linear increase in bulk density up to the fourth pass, after which additional passes have little effect with the soil having reached a maximum density.
Speed of working during cropping operations can also influence changes in soil bulk density. Slower speeds result in a longer time that the forces are applied to the soil, often referred to as the ‘dwell time’. A longer dwell time increases the degree (severity) of soil compaction.
Tyres, tracks and axle load
Axle load (the total load supported by one axle, in tonnes) is the first thing to be considered when avoiding soil compaction. This is because the depth of compaction increases with the increase of axle load. Weights of 6 tonne per axle will contribute to subsurface soil compaction. Research indicates that an axle load of 10 tonne will induce subsurface soil compaction to depths of 500 millimetres, or more.
Contact pressure (pressure exerted by tyre or track on the soil surface, expressed as kilograms per square centimetre) also has an influence on the amount of compaction induced. Therefore, tyre pressure has a large impact on the transfer of weight to depth. Lower tyre pressures ensure reduced surface compaction. The higher the pressure in the tyre, the greater pressure placed on the soil and depth to which it is applied. To limit compaction, tyre pressure should always be the lowest allowable for the procedure being conducted at the time. Adding ‘duals’ or ‘triples’ reduces the carrying load of each tyre and therefore reduces the near surface compaction. Duals can have similar compression forces to tracks, provided the tyre pressure is low.
The compaction caused by tyres and tracks differs slightly. Studies comparing wheeled and rubber tracked tractors indicate that tyred tractors result in a greater soil strength in the 200–400 millimetres soil depth. However, there is little difference in resulting soil strength between the two systems below 400 millimetres. For tracks, load is not equally distributed along track length. Spikes in load occur beneath each idle roller with the peak load under the rear drive roller. Each axle in a track, including idle rollers, represents a traffic pass over the soil. The load of the rear drive roller increases as tow load increases. While the contact pressure of tracks is lower than tyres, the dwell time is greater. Vibration, which reduces soil porosity as particles settle, is more readily transferred from the machine to the soil. Often lower compaction is measured under tracks, compared to tyres, though soil strength will still be high enough to restrict root growth.
Size, width and depth of soil compaction
Tyres, tracks and axle load all need to be considered when addressing soil compaction. From Soil Quality: 6 Soil Compaction (Parker et al. 2021). Voice talent: Wayne Parker, DPIRD; animation: Bee Box Marketing.
Impact of soil compaction
Compacted soil lacks the interconnected air spaces and has fewer large pores that are essential to the movement of water, gases and plant roots. Consequently, plant available water (PAW) which depends on both the available water capacity of the soil and the depth of rooting is affected by compaction. Because water is held more tightly within the smaller pores of a compacted soil and it is therefore more difficult to extract, plants do not grow so well in compacted soils. Root exploration of the soil is also inhibited.
The net effects of compaction are often reduced plant growth and crop yields, although if moisture and nutrients in the soil above the compacted layer are adequate, there may be no yield penalty in some seasons. The impact of compaction is particularly evident in a dry finish when plant roots are unable to penetrate compacted layers late in the season, restricting the plant’s access to stored moisture and nutrients during its growth and resulting in production losses.
Compacted soils can impact agricultural production in some or all of the following ways.
Lower rates of water infiltration, particularly at the soil surface
Decreased porosity slows water infiltration into the soil profile. Both frequency and intensity of rainfall will influence surface water loss and plant available water.
Decrease soil water storage
Compacted soil layers stop water re-filling the subsoil (clay soils only). The amount of plant available water can be halved when serious compaction occurs. Since water is unable to permeate the subsoil, the surface layer becomes saturated if rain continues and the increased run-off can result in a greater risk of erosion and loss of nutrients.
Increase incidence of waterlogging for clayey soils
Compacted soils have poor aeration and drainage. Where water is unable to penetrate compacted layers, slow infiltration and decreased permeability reduce deep drainage resulting in a greater risk of waterlogging.
Less favourable habitat for soil biota
Although soil biota can populate a wide range of environments, the majority function in an aerobic (aerated), well drained habitat. Compaction results in degradation of habitat for soil biota and may inhibit earthworms and microorganisms that are beneficial for the formation of biopores and plant growth.
Slow rates of plant root growth
Dense soil layers restrict plant roots growing to depth and accessing available water. The absence of roots at depth has significant implications on the crops ability to access moisture under drying conditions. The degree to which crop roots are affected will depend upon the extent and depth of compaction, crop species and climate.
Plant growth in compacted soil is primarily impacted in four ways:
- The ease with which roots can grow through the soil – greater force and energy required to penetrate the compacted layer.
- The rate at which roots grow – decreased number of macro-pores slows root growth; plants more susceptible to soil pathogens.
- The proliferation of roots – density and branching of roots decreases within the compacted zone, and below.
- Seedling emergence – lower plant establishment and smaller, less vigorous seedlings depending on soil type.
Decrease water and nutrient uptake
Hardpans (‘traffic pans’ in sands, ‘plough pans’ in clays) cause water and nutrients at depth to remain largely inaccessible as plant roots are unable to penetrate the compacted layer. This can result in poor nutrient and water use efficiency, poor tolerance to stress under drying conditions and declining yields.
How compacted soil impacts root growth and changes access to nutrients
This animation of a simulated model shows the impact of a compacted subsoil layer in the development of a plant root system (right), in comparison to root growth in a soil without compaction (left). From Soil Quality: 10 Plant Nutrition (Scanlan et al. 2023). Animation: Andrea Schnepf, Tobias Selzner and Sonja Habbinga, Forschungszentrum Jülich GmbH; script: Craig Scanlan, DPIRD; voice talent: Ryan Borrett, Murdoch University; background music: Izzy Video; video editing: Science with Style.
Diagnosing soil compaction
The extent and severity of soil compaction across the south-western agricultural region of Western Australia suggests that this constraint is often overlooked. Visual symptoms and physical measures that indicate the severity and depth of compaction are available and should be combined with other soil quality indicators to identify best management practice.
- Compaction can be difficult to diagnose as it occurs below the soil surface.
- Long-term cropping using machinery of varied wheel track widths often means the whole paddock is suffering compaction.
- Look for patterns in crop emergence and growth, particularly focussing on changes in more heavily trafficked areas.
- Use a penetrometer or hand probe in moist soil, and/or dig a hole to feel and observe more densely packed soil.
The impact of constrained root growth on crops resulting from compaction is not always evident and can be difficult to identify in the paddock. Extensive areas of the paddock may be affected and the extent of compaction gradually worsened through time – thus the whole crop is constrained and may look quite even in growth. Often the constraint becomes evident where a contrast is provided. For example, aggressive tillage associated with the burial of a water pipe across a paddock can generate large differences in crop growth with larger, healthier plants along the pipeline than those in the remainder of the paddock. Similarly, the removal of a fence where soil has remained untrafficked can highlight stark differences in plant growth.
While compaction can sometimes be diagnosed by visual observations of crop growth, it is often difficult to distinguish from other limiting factors such as root disease, spray damage, nutrient deficiency, soil acidity, waterlogging, sodicity or machinery faults at seeding, which may be responsible for poor crop growth. In-field investigation is necessary to determine cause.
Above-ground indicators of compaction include:
- poor plant germination and establishment, as well as retarded early growth (can indicate near-surface compaction)
- stunted and yellow appearance of plants within high traffic zones, even in the absence of visual wheel damage
- areas of delayed flowering in lupin or canola
- areas where crop and pasture growth appears water restricted during dry periods and hay-off more rapidly at the end of the season
- large dense clods of soil brought to the soil surface with deep seeding points (can indicate subsurface compaction)
- water ponding on the surface or waterlogging (may indicate near-surface or subsurface compaction and is associated with the slower infiltration and flow rate of water into and through soil).
Observational clues/diagnosis of compaction
A quick and easy tool is a metal rod pushed into the soil by hand when the soil is wet to identify at what depth it becomes more difficult to penetrate soil.
Digging a hole or soil pit is a useful way to visually observe soil and plant root characteristics. It also provides an opportunity to assess and diagnose other soil constraints that may be present, such as surface and subsurface soil acidity, aluminium toxicity, waterlogging and sodicity.
Compacted soil layers often have distinct upper and lower boundaries and a blocky appearance that can be identified visually. This may be less evident in heavier textured soil types or shallow duplexes where the dense clay layer is about 200 millimetres below the surface. However, a compacted layer will ‘feel’ denser and less friable (tighter) than the soil above or below it.
When pressure is applied to clods from the compacted layer they tend to fracture where the pressure is applied, not along natural break lines as happens with well-structured soil. Soil that crumbles into smaller aggregates along random paths in the clod is a good indicator of a well structured soil. In gravel soil, and sandy gravel in particular, compaction is signalled where the imprint of a gravel stone remains after the stone is prised from the profile.
Observing the patterns of root growth either within the face of the soil pit, or by digging out an intact plant, can provide an indication of the presence of compacted soil layers:
- In compacted soil, roots tend to proliferate along the surface of hard layers as they look for natural fractures or existing biopores, such as old root channels. At these points, there will be a large number of roots compared to the rest of the soil at this depth.
- Plants often have shallow root growth and swollen or stubby root tips, caused as roots grow through the hard soil. In poorer growth areas of a paddock, deformed and bent roots of individual plants can be evident when dug up, especially in canola, and can be strong evidence for subsurface compaction. However, root pruning is also a symptom of aluminium toxicity or soilborne diseases – so further investigation is required to determine the cause.
- Moist soil within 300–400 millimetres of the surface in cropped areas at the end of a season suggests that there have been insufficient roots at this depth to extract water and may indicate soil compaction. Other constraints can also result in poor water use an “poor water use and further investigation is required.
Physical indicators of compacted soil may be observed during tillage operations. For example, compact soil may be physically difficult for the tillage implement to penetrate, causing an increase in the draft force on the tractor. Such changes will alter fuel use patterns at seeding time – though be mindful of soil type changes within the paddock. Deeper tillage when the soil is dry or only slightly wet can bring up large dense clods from compacted layers.
Root symptoms of a compacted soil
Impact of compaction on root growth in different soil types
Diagnosing compaction in loamy soil
Changes in characteristic features, such as proliferation of roots and a shift from a crumbling soil to blocky, angular soil structure can indicate compaction. From Soil Quality: 6 Soil Compaction (Parker et al. 2021). Video talent: Wayne Parker, DPIRD; video production: Lomax Media.
Compaction is most accurately measured with a cone penetrometer, which provides a quantitative assessment of soil strength. The growth of plant roots starts to be restricted at 1.5 megapascals (1500 kPa) and is severely restricted at 2.5 megapascals (2500 kPa). However, more qualitative measures are available (e.g. using hand probes) that can identify the compaction layer sufficiently well to identify the presence or absence, and depth of compaction.
Bulk density may be a useful measure where soil texture doesn’t change down the soil profile. Soil with a higher bulk density occupies less volume per weight of soil (or alternately more soil weight per unit volume) – indicating lower porosity. A compacted layer has a higher bulk density than soil either above or below its position in the soil profile.
While a measure of bulk density is also useful to convert soil analytical values (e.g. concentration of nutrients or carbon) to a weight-per-hectare value for paddocks, soil probes or penetrometers are usually more convenient to diagnose the presence and depth of compaction in the paddock. Neither penetrometer, nor bulk density measurement is useful to identify compaction layers in gravel soil as the presence of gravel stones distort any results.
Cone penetrometers
Cone penetrometers are metal rods with a cone tip that have either a digital or mechanical gauge that provides an indication of soil strength by measuring the force required to push into the soil. Penetrometer readings are influenced by many factors, such as:
- Soil particle and pore size distribution, soil water content, particle shape and roughness, and interparticle bonding.
- Dimensions of the penetrometer tip and type of instrument. In many cases the digital cone penetrometer provides a relative value of soil strength rather than an exact value, particularly if the soil is being assessed at lower than field capacity.
- Penetrometer readings are greatly affected by soil water content. When taking measurements the soil should be as near to field capacity (drained upper limit) as possible. At lower soil moisture, higher strength measurements are recorded. If necessary, water should be applied in a confined perimeter to wet soil to depth prior to measurement. For example, building up soil in a ring to form a dam and filling with water, or using a large bucket or bin of water with holes in the bottom, and allowing the water to slowly infiltrate (a porous cloth underneath the bucket helps with even wetting).
- Factors such as height, weight, strength and experience of the user.
Penetrometers do not necessarily reflect the pressure or energy exerted by plant roots growing through soil. For example, a penetrometer cannot change direction to follow the path of least resistance through a soil. Roots, by comparison, often use preferred pathways, such as old root channels and biopores, navigating their way through layers of high strength soil. Thus the pressure exerted by a penetrometer can be two to five times greater than what is required by the root to grow through the soil. Further, a penetrometer has no capacity to minimise the friction with the soil in the same way roots do; root caps and root cap mucilage assist to reduce friction as a root grows through soil.
The data from a cone penetrometer provides a measure of the changes in soil strength with increasing soil depth, and indicates the severity and depth of compaction. The potential growth rate of roots slows as soil strength increases. The linkages between root growth rate and penetrometer reading are not exact as they vary with soil type, soil water content, structure and texture. However, there are accepted parameters around penetrometer-measured strength indicating where the growth of annual crop roots slow, or stop. With exceptions, the speed of root growth in crops and pastures slows by 80% at a measured soil strength of 2.5 megapascals and all ceases at 3 megapascals.
Hand probes (push probes)
Hand probes are a simple metal rod pushed into the ground by hand to provide an indication of compaction and are best used when soil is moist to depth. Compacted layers can be felt as the force required to push the rod at first increases (resistance) and then eases once it moves from the hard compacted layer to the less dense, soft soil layer below. These simple implements can provide a guide of the depth to the hard layer and its approximate size. It is a qualitative measure of compaction as it provides only the indication of hard layers without quantifying their strength.
Hand-held force gauge
Hand-held force gauges are penetrometers that measure soil strength to a shallow depth. In a soil pit, they can be pushed horizontally into the pit face and are useful for measuring soil strength down the profile.
Using penetrometers and hand probes to identify soil compaction
Testing needs to be done when the soil is sufficiently wet to depth - preferably at the drained upper limit, or field capacity. A dry layer in the soil may otherwise present as a hard layer, regardless of whether or not it is compacted. From Soil Quality: 6 Soil Compaction (Parker et al. 2021). Video talent: Glenn McDonald, DPIRD; video production: Lomax Media.
Managing soil compaction
Once diagnosed, there is a suite of practices and amelioration techniques to manage compaction. In many cases, compaction hardpans can be economically ameliorated. Once ameliorated, adopting appropriate agricultural management practices can minimise the reformation of the hardpan.
- Mechanical tillage is required to remove compaction in soil that doesn’t naturally shrink and swell, such as soil with a high sand component.
- Adding amendments, such as lime, gypsum or organic matter, if required, when using tillage will help address multiple constraints at the same time and, in some cases, can stabilise soil structure changes.
- Controlled traffic farming will minimise compaction from heavy cropping machinery and maintain the longevity of soil amelioration.
Where soil is compacted and loosening has to be carried out quickly, mechanical tillage can be used. Use soil pit inspections to define the nature of the compaction problem to ensure the depth of tillage is not excessive. If undertaking deep tillage, consider incorporating lime, gypsum or organic amendments to maintain soil structure.
Crop responses to mechanical remediation of compaction are dependent on soil type. Crops on uniform coarse-textured soil, including coarse and fine sand, loamy sand and sandy loam, respond well to deep mechanical soil fracturing. Crop responses on soil with a texture contrast (i.e. duplex soil) will depend on the depth of the sand layer over the contrast soil, with deeper sand more likely to respond positively. Grain yield responses on medium to fine textured soil, including hard-setting loamy sand to sandy loam, light sand clay loam, or finer textured soil are less likely.
Following amelioration of compaction, the application of nutrients, including nitrogen, needs to account for increased access to soil resources, improved nutrient use efficiency and greater crop potential.
Mechanical alleviation of compaction increases the risk of soil erosion. Surface roughness, tall stubble and biomass cover slow wind speed at the soil surface and reduce the risk of erosion. Deep ripping disrupts old roots and root crowns, which stabilise the surface soil, and the rolling process flattens stubble and crushes clods, reducing the roughness of the surface. Techniques such as ploughing have a greater impact, leaving soil bare and completely exposed. Establishing biomass cover as soon as possible mitigates the risk of erosion.
Subsidence and re-compaction limit the persistence of the benefits of deep tillage operations. The incorporation of gypsum (to prevent dispersion in sodic soils), lime and/or gypsum (in acidic sodic soils) or organic amendments should be considered to help stabilise soil and increase long term benefits after tillage. However, it is important to realise adding lime is of no value in high pH soils because the lime does not dissolve sufficiently. It is the calcium (Ca) ions that benefit the soil structure; hence gypsum which is much more soluble than lime should be used in this soil type.
Root channels or ‘biopores’ formed by soil fauna (e.g. earthworms, ants) are useful for loosening soil, the transport of water and gases in the profile, as well as ‘housing’ new roots. Adopting plant species with vigorous rooting systems can also result in improved soil aggregation and organic matter at depth, providing a suitable environment for beneficial soil fauna.
Primer plants are grown specifically for the benefit of the following crop by the root growth providing a channel or ‘preferred pathway’ through compacted layers. Ideally, the primer plant has a strong root system, able to penetrate dense subsoil and withstand adverse soil conditions that may also be present, such as salinity, to extend root growth. Plants with thick tap roots are better able to penetrate soil with high bulk density than fibrous root systems. The roots remaining after a primer plant phase provide pathways for following crops to grow through high strength. A no-tillage system is best able to maintain these biopores once formed. However if the compaction is too severe, the roots may grow sideways or push up out of the ground. Growing primer plants with a fine, dense root system helps to preserve soil porosity and stability after tillage.
Prevention of soil compaction
Prevention of soil compaction is better than the cure. However, already compacted soils can be remediated by several management practices, or this may occur naturally. Depending on the degree of compaction, the need for practice change may consider the ability of a soil or farming system to ‘naturally repair’; otherwise some form of mechanical intervention may be necessary.
Options exist to prevent or minimise the risk of soil compaction:
- Minimise traffic when soil is wet and most susceptible to compaction.
- Keep tyre pressures as low as practically possible.
- Use dual or triple wheels, or very wide tyres (1200 mm) to minimise wheel load and decrease surface compaction – deep subsurface compaction will still occur.
- Restrict machinery traffic to permanent wheel tracks (also called tramlines or traffic lanes) in a controlled traffic farming system to minimise impacted area.
Controlled traffic farming systems minimise the area of the paddock that is compacted by heavy cropping machinery by establishing and maintaining permanent pathways for machinery. A fully-matched controlled traffic farming system, where tracks and machinery widths match across tractors, spray rigs and other required machinery, will traffic between 9 and 12% of the paddock. This compares to other farming systems in Western Australia in which approximately 45% of the paddock is trafficked in any one season; and where, with no traffic control, can accumulate to 100% of the paddock wheeled over a few seasons.
The implementation of controlled traffic is an essential partner to soil loosening in minimising or repairing compaction. The land area affected by compaction due to machinery movement can be minimised by confining all or most tillage and traffic operations to designated vehicle pathways where possible.
If controlled traffic is not an option then a reduction in traffic across paddocks at harvest, by restricting field bin and truck movement to the edge of the paddock can also alleviate compaction.
Soil texture and organic matter content of the topsoil are important in determining the ability of soil to recover from compaction. In soils with less than 1.7% organic carbon an indication of the shrink-swell potential can be gained by measuring the cation exchange capacity (CEC) of the soil if the clay percentage is known.
Annual inputs of organic matter should be maximised through appropriate agronomic management of crop and pasture systems. Tillage strategies that reduce soil disturbance and maximise organic matter accumulation (i.e. stubble retention) can be used to influence soil compaction by altering organic matter decomposition and protecting the soil surface.
Decreasing the number and degree of tillage operations during high risk periods (i.e. when soil is wet) will lessen the risk of compaction developing.
In dry compacted soils, tillage fractures and loosens the soil and can actually benefit some soils (e.g. cracking clays). However, clay-rich soil is prone to damage if tilled when wet, resulting in compaction and/or remoulding which may aggravate waterlogging and dispersion problems – avoid excessive traffic and tillage at high risk times. On loamy soil, the optimal water content for tillage is just below the plastic limit . In very sandy soil, the water content at which tillage is carried out is less critical than on soil with a heavier texture.
Reduce the number of tillage operations, or adopt zero tillage. In some crops, cultivation can be restricted to strategic areas such as the plant rows (e.g. ploughout in sugarcane) or the edges of beds (“shoulder busting” by cotton growers). Trials have shown improved moisture storage can improve yield by up to 15 to 25% under zero-tillage compared to cultivation (provided nutrient supply is adequate). Where tillage is necessary, varying the depth of cut between operations or seasons can help prevent formation of a ‘plough pan’.
Near-surface soil compaction is inevitable with grazing livestock. However, when the soil is moist and most prone to damage, stock can be moved to parts of the farm where there is less risk of soil compaction or where it is most easily rectified at a later date. Removing or reducing stock (particularly cattle) when the soil is wet will help reduce the risk of compaction, as will maintaining high stocking rates for shorter duration.
Integrated management
Soil compaction is rarely identified in the absence of other soil constraints, either in the surface or subsurface. Consideration must then be given to which constraints throughout the profile, such as soil acidity, sodicity and water repellence, present the greatest constraint to maximising the response when managing for compaction. This helps determine the most appropriate method of amelioration and prevents hostile soil being brought to the surface.
It is advisable to target the most limiting factor and add value by including treatment of other constraints where able. To do this most effectively, it is necessary to understand the likely return on investment over both the short- and long-term.
Where compaction is identified as a constraint in sandy soil, it should be considered as the first priority for management in conjunction with ameliorating constraints that will impede plant root growth, such as soil acidity. Reasons for this include:
- Deep ripping is accessible and cost efficient with capacity to ameliorate large areas in one season.
- Yield increases of between 10 and 40% provide potential for a large return on investment.
- Growers have found deep ripping prior to spading improves soil inversion, burying topsoil much deeper and more evenly than spading unripped, compacted soil.
- With the development of new machines, it is possible to use one machine to deep rip and spade in one pass, addressing multiple constraints in a single operation.
The return on investment from managing compaction is able to assist in payment for amelioration of other constraints. There is the challenge of compromise when managing more than one constraint. Each method of amelioration has strengths and weaknesses that determine the financial and physical outcome. Decisions around which tool to use are based on cost of implementation and the likely result of the amelioration. Ranking Options for Soil Amelioration (ROSA) is a tool available from the Department of Primary Industries and Regional Development to assist growers and consultants with the decisions as targeted to their budget and soil types.
Spading, mouldboard ploughing, one-way ploughing and off-set disc ploughing have the capacity to ameliorate compaction to their respective working depth (<400 mm), while managing other constraints at the same time (e.g. soil water repellence and subsoil acidity). For example, spading a paddock with surface water repellence, subsoil compaction and an acidic subsoil can target amelioration of all constraints to differing degrees. Spading mixes the topsoil through the profile to a depth of up to 400 millimetres and, in doing so, the spader breaks compacted soil to the depth of working. Surface-applied lime can be spaded into the subsurface soil to ameliorate acidity and can alleviate surface water repellence both by burying the repellent layer below the surface and by bringing subsurface clay to the surface – effectively decreasing the surface tension. If the compaction layer is deeper than the working depth, a separate deep ripping operation is required to remove the residual compaction.
Assessment of the extent and severity of soil constraints present will determine the likely benefit of combining amelioration strategies.
Economics
The economics of ameliorating soil compaction depend on the individual situation, soil type and severity of compaction, land use and the management options available.
Consider the following checklist when determining appropriate management:
- Confirm soil is compacted and identify depth of compacted layer
- Identify extent of the problem (affected area)
- Seek confirming data (yield and biomass maps, penetrometer readings)
- Be aware of other subsoil constraints that may limit success of mechanical intervention or that could be addressed at the same time, such as sodic or acidic subsoils.
- Ask yourself – is this my most limiting factor?
The economic basis for ameliorating soil compaction is strongest where the employment of management practices that support the natural repair of soil structure is possible, such as stubble retention to increase organic matter or incorporating deep rooted crop species in rotation. One of the factors against the adoption of management practices that support natural repair methods such as zero-tillage, stubble cover, controlled traffic on soil compaction is the length of time sometimes required for responses to be observed.
Davies S, Parker W, Blackwell P, Isbister B, Betti G, Gazey C and Scanlan C (2017). Soil Amelioration in Western Australia. Paper presented at 2017 GRDC Grains Research Updates, Perth. [online]
Page references and acknowledgements
Material on this page adapted from:
- Hoyle FC (2007). Soil Health Knowledge Bank.
- Parker W, Isbister B, Hoyle FC and Leopold M (2021). Soil Quality: 6 Soil Compaction. SoilsWest, Perth, Western Australia. [Access]
Last updated July 2024.
